Methods and apparatus for computing volumetric perfusion

ABSTRACT

A method for computing volumetric perfusion in a spatially stationary organ using a computed tomography (CT) imaging system includes positioning an area detector such that the area detector encompasses a spatially stationary organ within the field of view of the imaging system for all view angles, operating the CT imaging system in a cine mode to acquire a plurality of projection data representative of the tissue dynamics in the spatially stationary organ, processing the projection data, and computing the volumetric perfusion in the organ using the reconstructions of the projection data representative of the tissue dynamics.

BACKGROUND OF THE INVENTION

This invention relates generally to computed tomography (CT) imaging andmore particularly to an apparatus and method for computing volumetricperfusion from temporal reconstructions of tissue attenuationcharacteristics using digital area detector technology.

In at least one known “third generation” CT system, projection data iscontinually acquired within a limited axial coverage of a patient toadequately measure the uptake and washout of a contrast medium in anorgan being imaged. Additionally, as many as sixteen slices ofprojection data to be acquired, reconstructed, and processedsimultaneously for perfusion evaluation can be accomplished usingcurrent multi-row detectors. The scanning speed of a known CT system isadequate for sampling of the contrast dynamics of the tissue within asmall volume of an organ; however, the scanning speed is inadequate forsampling the contrast dynamics for a whole organ being imaged, such as abrain since helical scanning protocols are necessary.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a method for computing volumetric perfusion in aspatially stationary organ using a computed tomography (CT) imagingsystem is provided. The method includes positioning an area detectorsuch that the area detector encompasses a spatially stationary organwithin the field of view of the imaging system for all view angles,operating the CT imaging system In a cine mode to acquire a plurality ofprojection data representative of the tissue dynamics in the spatiallystationary organ, processing the projection data, reconstructing theprojection data, and computing the volumetric perfusion in the organusing the reconstructed projection data representative of the tissuedynamics.

In another aspect, a computed tomography (CT) imaging system forcomputing volumetric perfusion in a spatially stationary organ isprovided. The CT imaging system includes a radiation source, an areadetector, and a computer operationally coupled to the radiation sourceand the area detector. The computer is configured to position an areadetector such that the area detector encompasses a spatially stationaryorgan within the field of view of the imaging system for all viewangles, operate the CT imaging system in a cine mode to acquire aplurality of projection data representative of the tissue dynamics inthe spatially stationary organ, processing the projection data,reconstructing the projection data, and compute the volumetric perfusionin the organ using the reconstructed projection data representative ofthe tissue dynamics.

In a further aspect, a computer readable medium encoded with a programis provided. The medium is configured to instruct a computer to positionan area detector such that the area detector encompasses a spatiallystationary organ within the field of view of the imaging system for allview angles, operate the CT imaging system in a cine mode to acquire aplurality of projection data representative of the tissue dynamics inthe spatially stationary organ, process the projection data, reconstructthe projection data, and compute the volumetric perfusion in the organusing the reconstructions of the projection data representative of thetissue dynamics.

In still another further aspect, a method for obtaining data of aspatially stationary organ using a Computed Tomography (CT) imagingsystem having a field of view is provided. The method includespositioning an area detector such that the area detector encompasses aspatially stationary organ within the field of view of the imagingsystem for all view angles, operating the CT imaging system in a cinemode to acquire a plurality of projection data representative of thespatially stationary organ, and filtering the acquired projection datato provide data with an improved signal-to-noise ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a flowchart illustrating a method for computing volumetricperfusion in a spatially stationary organ using a computed tomography(CT) imaging system.

FIG. 4 is a flow chart illustrating a method for obtaining data of aspatially stationary organ using a Computed Tomography (CT) imagingsystem.

DETAILED DESCRIPTION OF THE INVENTION

The methods and apparatus described herein address acquiring projectiondata allowing improved computation of the volumetric perfusion in anorgan using area detector technology. Volumetric perfusion refers to thecomputation of mean transit time, blood volume, and/or blood flow fromreconstructions of projection data, or the combination of any of thesequantities into one mathematical metric. Methods are described tofacilitate reducing CT imaging system constraints such as a gantryscanning speed. In addition, the methods described herein facilitatereducing noise and improving the temporal resolution relative to thegantry scanning speed in contrast enhanced measurements, therebyimproving the stability and accuracy of a deconvolution process used inperfusion computations.

In some known CT imaging system configurations, an x-ray source projectsa fan-shaped beam which is collimated to lie within an X-Y plane of aCartesian coordinate system and generally referred to as an “imagingplane”. The x-ray beam passes through an object being imaged, such as apatient. The beam, after being attenuated by the object, impinges uponan array of radiation detectors. The intensity of the attenuatedradiation beam received at the detector array is dependent upon theattenuation of an x-ray beam by the object. Each detector element of thearray produces a separate electrical signal that is a measurement of thebeam intensity at the detector location. The intensity measurements fromall the detectors are acquired separately to produce a transmissionprofile.

In third generation CT systems, the x-ray source and the detector arrayare rotated with a gantry within the imaging plane and around the objectto be imaged such that the angle at which the x-ray beam intersects theobject constantly changes. A group of x-ray attenuation measurements,i.e., projection data, from the detector array at one gantry angle isreferred to as a “view”. A “scan” of the object comprises a set of viewsmade at different gantry angles, or view angles, during one revolutionof the x-ray source and detector about the object to be imaged.

In an axial scan, the projection data is processed to construct an imagethat corresponds to a two-dimensional slice taken through the object.One method for reconstructing an image from a set of projection data isreferred to in the art as the filtered back-projection technique. Thisprocess converts the attenuation measurements from a scan into integerscalled “CT numbers” or “Hounsfield units”, which are used to control thebrightness of a corresponding pixel on a cathode ray tube display.

To reduce the total scan time, a “helical” scan may be performed. Toperform a “helical” scan, the patient is moved while the data for theprescribed number of slices is acquired. Such a system generates asingle helix from a fan beam helical scan. The helix mapped out by thefan beam yields projection data from which images in each prescribedslice may be reconstructed.

Reconstruction algorithms for helical scanning typically use helicalweighing algorithms that weight the collected data as a function of viewangle and detector channel index. Specifically, prior to a filteredbackprojection process, the data is weighted according to a helicalweighing factor, which is a function of both the gantry angle anddetector angle. The weighted data is then processed to generate CTnumbers and to construct an image that corresponds to a two-dimensionalslice taken through the object.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly recited.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features.

Also as used herein, the phrase “reconstructing an image” is notintended to exclude embodiments of the present invention in which datarepresenting an image is generated but a viewable image is not. However,many embodiments generate (or are configured to generate) at least oneviewable image.

Referring to FIGS. 1 and 2, a multi-slice scanning imaging system, forexample, a computed tomography (CT) imaging system 10, is shown asincluding a gantry 12 representative of a “third generation” CT imagingsystem. Gantry 12 has an x-ray source 14 that projects a beam of x-rays16 toward a detector array 18 on the opposite side of gantry 12.Detector array 18 is formed by a plurality of detector rows (not shown)including a plurality of detector elements 20 which together sense theprojected x-rays that pass through an object, such as a medical patient22. Each detector element 20 produces an electrical signal thatrepresents the intensity of an impinging x-ray beam and hence allowsestimation of the attenuation of the beam as it passes through object orpatient 22 when compared to the electrical signal that is measured whenno patient is placed in gantry 12. During a scan to acquire x-rayprojection data, gantry 12 and the components mounted thereon rotateabout a center of rotation 24. FIG. 2 shows only a single row ofdetector elements 20 (i.e., a detector row). However, multislicedetector array 18 includes a plurality of parallel detector rows ofdetector elements 20 such that projection data corresponding to aplurality of quasi-parallel or parallel slices can be acquiredsimultaneously during a scan. Moreover, an area detector array 18includes many rows of detector elements 20 such that projection datacorresponding to large volume can be acquired simultaneously during ascan.

Rotation of gantry 12 and the operation of x-ray source 14 are governedby a control mechanism 26 of CT system 10. Control mechanism 26 includesan x-ray controller 28 that provides power and timing signals to x-raysource 14 and a gantry motor controller 30 that controls the rotationalspeed and position of gantry 12. A data acquisition system (DAS) 32 incontrol mechanism 26 samples analog data from detector elements 20 andconverts the data to digital signals for subsequent processing. An imagereconstructor 34 receives sampled and digitized x-ray data from DAS 32and performs high-speed image reconstruction. The reconstructed image isapplied as an input to a computer 36, which stores the image in a massstorage device 38. Image reconstructor 34 may be specialized hardware orsoftware operating on computer 36.

Computer 36 also receives commands and scanning parameters from anoperator via console 40 that has a keyboard. An associated cathode raytube display 42 allows the operator to observe the reconstructed imageand other data from computer 36. The operator supplied commands andparameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28, and gantry motor controller30. In addition, computer 36 operates a table motor controller 44, whichcontrols a motorized table 46 to position patient 22 in gantry 12.Particularly, table 46 moves portions of patient 22 through gantryopening 48.

In one embodiment, computer 36 includes a device 50, for example, afloppy disk drive or CD-ROM drive, for reading instructions and/or datafrom a computer-readable medium 52, such as a floppy disk or CD-ROM. Inanother embodiment, computer 36 executes instructions stored in firmware(not shown). Computer 36 is programmed to perform functions describedherein, and as used herein, the term computer is not limited to justthose integrated circuits referred to in the art as computers, butbroadly refers to computers, processors, microcontrollers,microcomputers, programmable logic controllers, application specificintegrated circuits, and other programmable circuits, and these termsare used interchangeably herein. Additionally, although described in amedical setting, it is contemplated that the benefits of the inventionaccrue to all CT systems including industrial CT systems such as, forexample, but not limited to, a baggage scanning CT system typically usedin a transportation center such as, for example, but not limited to, anairport or a rail station.

FIG. 3 is a flow chart illustrating a method 60 for computing volumetricperfusion in a spatially stationary organ using computed tomography (CT)imaging system 10. Method 60 includes positioning 62 an area detector 18such that area detector 18 encompasses a spatially stationary organwithin the field of view of the imaging system for all view angles,operating 64 the CT imaging system in a cine mode to acquire a pluralityof projection data representative of the tissue dynamics in thespatially stationary organ, generating 66 reconstructions of thecontrast dynamics of the tissue using the projection data, and computing68 the volumetric perfusion in the organ using the reconstructions ofthe projection data representative of the tissue dynamics.

In use, a spatially stationary organ 22, such as a brain, is positionedbetween radiation source 14 and area detector 18 such that area detector18 encompasses a spatially stationary organ within the field of view ofthe imaging system for all view angles, i.e. area detector 18 ispositioned to encompass a field of view of brain 22 for all view angles.In one embodiment, a plurality of signals emitted from area detector 18are digitized, and a plurality of rows of area detector 18 are digitizedsimultaneously such that a sampling frequency of area detector 18readout is, for example, 120 frames per second. In some area detectorreadout schemes, if 4 rows of detector data 18 are simultaneouslymultiplexed into digitizing electronics, 960 views of spatiallystationary organ 22, can be acquired per rotation in a 2 second scan.System 10 is operated in a cine mode such that the sampling frequency ofthe projection data measuring the uptake and washout of a contrast agentin spatially stationary organ 22 at each view position is approximately1 second when gantry 12, including area detector 18, rotates at a speedof approximately 1 rotation per second.

Accordingly, using sampling theory and the assumption that theprojection data at a particular position of gantry 12 have low frequencycontent and do not violate the Nyquist sampling theorem, the projectiondata at any instant of time for each of the view angles can be computedand then reconstructed. Therefore, the contrast dynamics in spatiallystationary organ 22 being imaged can be computed at any instant of timeusing the interpolated projection data at each view angle position for ascan. In one embodiment, method 60 is used to itemize the processingsteps for at least one known perfusion algorithm. Additionally,volumetric perfusion analysis can be accomplished since area detector 18measures the projection data for the complete spatially stationary organ22 simultaneously. In one embodiment, the sampled projection data aretemporally filtered and interpolated to any instant of time, therebyfacilitating a reduction in noise and an improvement in the temporalresolution of the contrast dynamics using signal processing techniquessince a plurality of acquisitions are used to generate the projectiondata at the particular instance in time.

In one embodiment, a speed of gantry 12 is reduced and the processingsteps implemented, thereby facilitating an increase in the views usedfor reconstruction. In another embodiment, the views for reconstructioncan be increased by reducing an axial coverage of area detector 18 bydigitizing a fewer quantity of rows, less than the first quantity ofrows. In another embodiment, the views for reconstruction can beincreased by reducing the axial resolution of area detector 18 bymultiplexing and digitizing a quantity of rows greater than the firstquantity of rows of area detector data 18.

In an exemplary embodiment, system 10 facilitates computation ofvolumetric perfusion measurements that include an increased temporalresolution in the reconstruction of contrast dynamics, and an increasedsignal-to-noise ratio in the projection data, thereby improving theimage quality in reconstructed images. System 10 also facilitates anincrease in the axial coverage for perfusion computation by using areadetector technology. Additionally, projection data are interpolated to aparticular instant in time, therefore, temporal averaging of projectiondata in CT reconstructions is minimized. The signal processing used inthe interpolation process to resolve contrast dynamics in projectionimages also facilitates a reduction in the noise measurements.

FIG. 4 is a flow chart illustrating a method 80 for obtaining data of aspatially stationary organ using a Computed Tomography (CT) imagingsystem having a field of view. Method 80 includes positioning 82 an areadetector such that the area detector encompasses a spatially stationaryorgan within a field of view for all view angles, operating 84 the CTimaging system in a cine mode to acquire a plurality of projection datarepresentative of the spatially stationary organ, temporally filtering86 the acquired projection data to provide data with an improvedsignal-to-noise ratio, interpolating 88 the projection data to anyinstant in time, reconstructing 90 the filtered, time-resolvedprojection data, and computing 92 the volumetric perfusion in the organusing the reconstructions of the projection data representative of thetissue dynamics. Method 80 also includes filtering the acquiredprojection data at a selected frequency range. Either of the steps oftemporally filtering 86 or interpolation 88 in method 80 may beoptionally omitted depending on a certain imaging application.

Accordingly, system 10 enables enhanced volumetric perfusionmeasurements, improves temporal resolution in reconstruction of contrastdynamics of human tissue, and improves the signal-to-noise in projectiondata, thereby improving image quality in reconstruction, and improvingperfusion estimates in human tissue.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims. Although specific mention of third generation CT imaging systemsis incorporated herein, a fourth generation CT system (stationarydetector and rotating x-ray source) and a fifth generation CT system(stationary detector and stationary x-ray source) can be used toimplement the method, imaging system, and computer readable mediumencoded with a program. The signal processing methods described hereinmay also be used with existing CT systems to improve the signal-to-noiseratio in projection data and improve the temporal resolution of thecontrast dynamics, thereby improving the image quality in reconstructedimages of the contrast dynamics within an organ, such as the brain.These methods can be used to reduce the dose of ionizing radiationadministered to a patient to improve patient safety, while achieving thesame image quality in reconstructed images of the contrast dynamics intissue within the patient.

1. A method for computing volumetric perfusion in a spatially stationaryorgan using a computed tomography (CT) imaging system having a field ofview, said method comprising: positioning an area detector such that thearea detector encompasses a spatially stationary organ within the fieldof view of the imaging system for all view angles; operating the CTimaging system in a cine mode to acquire a plurality of projection datarepresentative of tissue dynamics in the spatially stationary organ;generating reconstructions of contrast dynamics of tissue using theprojection data; and computing the volumetric perfusion in the organusing the reconstructions of the projection data representative of thetissue dynamics.
 2. A method in accordance with claim 1, comprisingmultiplexing a first quantity of rows of the area detector such that asampling frequency of the area detector is increased.
 3. A method inaccordance with claim 1, wherein positioning the area detector such thatthe area detector encompasses the spatially stationary organ within thefield of view of the imaging system comprises positioning the areadetector such that the area detector encompasses a field of view of abrain.
 4. A method in accordance with claim 1, wherein operating the CTimaging system in the cine mode to acquire a plurality of projectiondata representative of the tissue dynamics in the spatially stationaryorgan comprises operating the CT imaging system in the cine mode toacquire the plurality of projection data representative of the tissuedynamics in the spatially stationary organ in which a contrast agent hasbeen introduced such that the projection data measuring an uptake andwashout of the contrast agent in the spatially stationary organ at eachview position is sampled at a suitable frequency.
 5. A method inaccordance with claim 1, further comprising multiplexing a quantity ofrows of the area detector.
 6. A method in accordance with claim 1,further comprising interpolating the plurality of projection data to aparticular instant in time, thereby enabling generation ofreconstructions with improved temporal resolution.
 7. A method inaccordance with claim 1, further comprising filtering the projectiondata at each view angle to reduce noise, thereby enabling generation ofreconstructions with improved image quality.
 8. A method in accordancewith claim 7, wherein filtering the projection data at each view angleto reduce noise allows a reduction in a radiation dose applied to apatient.
 9. A method for computing volumetric perfusion in a spatiallystationary organ using a computed tomography (CT) imaging system havinga field of view, said method comprising: positioning an area detectorsuch that the field of view of the area detector encompasses thespatially stationary organ for all view angles; operating the CT imagingsystem in a cine mode to acquire a plurality of projection datarepresentative of tissue dynamics in the spatially stationary organ suchthat the projection data measuring an uptake and washout of a contrastagent at each view position is sampled at a suitable frequency;generating reconstructions of contrast dynamics of tissue using theprojection data; and computing the volumetric perfusion in the organusing the reconstructions of the projection data representative of thetissue dynamics.
 10. A computed tomography (CT) imaging system forcomputing volumetric perfusion in a spatially stationary organcomprising: a radiation source; an area detector; and a computeroperationally coupled to said radiation source and said area detector,said computer configured to: position an area detector such that thearea detector encompasses the spatially stationary organ within a fieldof view of the imaging system for all view angles; operate the CTimaging system in a cine mode to acquire a plurality of projection datarepresentative of tissue dynamics in the spatially stationary organ;generate reconstructions of contrast dynamics of tissue using theprojection data; and compute the volumetric perfusion in the organ usingthe projection data representative of the tissue dynamics.
 11. A CTimaging system in accordance with claim 10, wherein said computer isfurther configured to multiplex a first quantity of rows of the areadetector such that a sampling frequency of the area detector increases.12. A CT imaging system in accordance with claim 10, wherein to positionthe area detector such that the area detector encompasses the spatiallystationary organ within the field of view of the imaging system, saidcomputer is further configured to position the area detector such thatthe area detector encompasses a field of view of a brain.
 13. A CTimaging system in accordance with claim 10, wherein to operate the CTimaging system in a cine mode to acquire a plurality of projection datarepresentative of the tissue dynamics in the spatially stationary organcomprises operating the CT imaging system, said computer is furtherconfigured to operate the CT system in the cine mode to acquire theplurality of projection data representative of the tissue dynamics inthe spatially stationary organ in which a contrast agent has beenintroduced such that projection data measuring an uptake and washout ofthe contrast agent in the spatially stationary organ at each viewposition is sampled at a suitable frequency.
 14. A CT imaging system inaccordance with claim 10, wherein said computer is further configured tomultiplex a quantity of rows of the area detector.
 15. A CT imagingsystem in accordance with claim 10, wherein said computer is furtherconfigured to interpolate the plurality of projection data to aparticular instant in time, thereby enabling generation ofreconstructions with improved temporal resolution.
 16. A CT imagingsystem in accordance with claim 10, wherein said computer is furtherconfigured to filter the projection data at each view angle to reducenoise, thereby enabling generation of reconstructions with improvedimage quality.
 17. A CT imaging system in accordance with claim 16,wherein filtering the projection data at each view angle to reduce noiseallows a reduction in a radiation dose applied to a patient.
 18. Acomputed tomography (CT) imaging system for computing volumetricperfusion in a spatially stationary organ comprising: a radiationsource; an area detector; and a computer operationally coupled to saidradiation source and said area detector, said computer configured to:position the area detector such that the area detector encompasses afield of view including the spatially stationary organ for all viewangles; and operate the CT imaging system in a cine mode to acquire aplurality of projection data representative of tissue dynamics in thespatially stationary organ such that projection data measuring an uptakeand washout at each view position is sampled at a suitable frequency.19. A computer readable medium encoded with a program configured toinstruct a computer to: position an area detector such that the areadetector encompasses a spatially stationary organ within a field of viewof the imaging system for all view angles; operate a computed tomography(CT) imaging system in a cine mode to acquire a plurality of projectiondata representative of tissue dynamics in the spatially stationaryorgan; generate reconstructions of contrast dynamics of tissue using theprojection data; and compute a volumetric perfusion in the organ usingthe reconstructions of the projection data representative of tissuedynamics.
 20. A computer readable medium in accordance with claim 19,further encoded to instruct the computer to multiplex a first quantityof rows of the area detector such that a sampling frequency of the areadetector increases.
 21. A computer readable medium in accordance withclaim 19, further encoded to instruct the computer to position the areadetector such that the area detector encompasses a field of view of abrain.
 22. A computer readable medium in accordance with claim 19,further encoded to instruct the computer to operate the CT system in acine mode to acquire a plurality of projection data representative ofthe tissue dynamics in the spatially stationary organ in which acontrast agent has been introduced such that the projection datameasuring an uptake and washout of the contrast agent in the spatiallystationary organ at each view position is sampled at a suitablefrequency.
 23. A computer readable medium in accordance with claim 19,further encoded to instruct the computer to multiplex more than one rowof the area detector.
 24. A computer readable medium in accordance withclaim 19, further encoded to instruct the computer to interpolate theplurality of projection data to a particular instant in time, therebyenabling generation of reconstructions with improved temporalresolution.
 25. A computer readable medium in accordance with claim 19,further encoded to filter the projection data at each view angle toreduce noise, thereby enabling generation of reconstructions withimproved image quality.
 26. A computer readable medium in accordancewith claim 25, wherein filtering the projection data at each view angleto reduce noise allows a reduction in a radiation dose applied to apatient.
 27. A method for obtaining data of a spatially stationary organusing a computed tomography (CT) imaging system having a field of view,said method comprising: positioning an area detector such that the areadetector encompasses a spatially stationary organ within a field of viewfor all view angles; operating the CT imaging system in a cine mode toacquire a plurality of projection data representative of the spatiallystationary organ; filtering the acquired projection data to provide datawith an improved signal-to-noise ratio; and interpolating the acquiredprojection data to generate projection data at any instant in time. 28.A method in accordance with claim 27, wherein said filtering theacquired projection data comprises filtering the acquired projectiondata at a selected frequency.